**9. A1c, Can it replace OGTT?**

The measurement of A1c equals the assessment of hundreds (virtually thousands) of fasting glucose levels and also capture postprandial glucose peaks; therefore, it is a more and reliable measurement than FPG and/or 2-hr OGTT plasma glucose (PG) oscillates above and below the cut point of 200 mg/dl.

FPG of 6.7 mmol/L to 7.2 mmol/L (120 or 130 mg/dl) or having a 2-h PG of 10.3 mmol/L to 11.9 mmol/L (185 or 215 mg/dl) are considered similar because they define a point where physiological disturbance is apparent, however from other angles it makes a lot of difference. Therefore, an appliance evaluating chronic rather than spot hyperglycaemia is unquestionably more desirable. A1c assay is now the preferred test not only for chronic management of diabetes but also for its diagnosis. However, the cost of assay in some parts of the world rules out its typical *Oral Glucose Tolerance Test (OGTT): Undeniably the First Choice Investigation… DOI: http://dx.doi.org/10.5772/intechopen.96549*

use. In such instances, clinicians should continue using glucose measurements for both diagnosis and monitoring of diabetes.

A1c assay may not be reliable under the underlisted conditions [115]


The glycated haemoglobin (HbA1c) test has been suggested as an alternative screening test for type 2 diabetes. HbA1c overcomes many of these difficulties as fasting state is not required, analytical variability is less than 2% and gives glycaemic status over past 2–3 month. The coefficient of variation is usually 2–3% for the same day analysis, while the inter-assay variation is 4–5%. HbA1c values are relatively stable after collection, and the recent introduction of a new reference method to calibrate all HbA1c assay instruments should further improves HbA1c assay standardization.

Advantages of HbA1c assay are:


Regrettably, at variance with that report, the New Hoorn Study [118] showed that 44% of people with newly diagnosed diabetes with OGTT had A1c < 6.0% and that a stronger correlations between plasma glucose and A1c is better in subjects with known diabetes, but not in the general population. Moreover, in the Rancho Bernardo Study [119], 85% of the participants with A1c ≥6.5% were not classified as diabetes by ADA criteria and a 1/3rd of people with diabetes on OGTT had A1c

**Table 13.**

reached odds ratio 1.75. These arbitrary thresholds, when applied to the HAPO cohorts, led to a GDM incidence of 17.8%. In 2013 Canadian Diabetic Association expert committee conceded the dispute and chosen sequential screening with a 50 g GCT followed by 75 g OGTT using the glucose thresholds that result in an Odds

Hyperglycaemia first detected at any time during pregnancy should be classified

When glucose abnormalities persist postpartum in a woman with GDM, her diabetes is re-categorized as overt diabetes, especially if the diagnosis of GDM occurred before 20 weeks' gestation and glucose levels were markedly elevated in pregnancy. The 2006 WHO criteria should be used in diagnosis of Diabetes mellitus

Ratio (OR) of 2.00 (fasting ≥5.3 mmol/L, 1 hour ≥10.6 mmol/L, 2 hours

in pregnancy when one or more of the following criteria are met:

• FPG ≥ 7.0 mmol/L (126 mg/dl), demonstrated on two occasions

• 2hPG ≥ 11.1 mmol/L (200 mg/dl) following a 75 g oral glucose load

• RPG ≥ 11.1 mmol/L (200 mg/dl) in presence of diabetic symptoms

The diagnosis of GDM at any time during pregnancy should be based on any one

There are no established criteria for the diagnosis of diabetes based on the 1-hour post-load value. At least one of these thresholds must be equaled or exceeded to

The measurement of A1c equals the assessment of hundreds (virtually thousands) of fasting glucose levels and also capture postprandial glucose peaks; therefore, it is a more and reliable measurement than FPG and/or 2-hr OGTT plasma glucose (PG) oscillates above and below the cut point of

FPG of 6.7 mmol/L to 7.2 mmol/L (120 or 130 mg/dl) or having a 2-h PG of 10.3 mmol/L to 11.9 mmol/L (185 or 215 mg/dl) are considered similar because they define a point where physiological disturbance is apparent, however from other angles it makes a lot of difference. Therefore, an appliance evaluating chronic rather than spot hyperglycaemia is unquestionably more desirable. A1c assay is now the preferred test not only for chronic management of diabetes but also for its

diagnosis. However, the cost of assay in some parts of the world rules out its typical

≥9.0 mmol/L).

• Diabetes mellitus in pregnancy

*Type 2 Diabetes - From Pathophysiology to Cyber Systems*

• Gestational diabetes mellitus

of the following values:

make a diagnosis of GDM.

200 mg/dl.

**138**

**9. A1c, Can it replace OGTT?**

• FPG 5.1–6.9 mmol/l (95–125 mg/dl)

• 1hPG 75 g OGTT ≥10.0 mmol/L (180 mg/dl)

• 2-hPG 75 g OGTT 8.5–11.0 mmol/L (153-199 mg/dl)

as either:

<sup>1.</sup> Assay is normalized and aligned to the DCCT/UKPDS

<sup>2.</sup> Better summary of overall glycaemic exposure and risk for long-term complications

<sup>3.</sup> It has significantly less biologic fluctuation

<sup>4.</sup> No need for fasting or timed samples

<sup>5.</sup> Relatively unaffected by acute(eg stress or illness related) perturbations in glucose levels

<sup>6.</sup>Currently used to guide management and adjust therapy

*The beauty of A1c testing compared to blood glucose for diagnose of diabetes mellitus.*

< 6.0%. Thus, this study demonstrated that 30% of subjects who are already diabetic or pre-diabetic would have been missed if A1c had been used instead of OGTT. In conclusion, the data confirmed, in agreement with an Australian study, that an A1C ≤5.5% or ≥ 7% can predict the absence or presence of diabetes respectively, while intermediate values are inconclusive (**Table 13**).

**References**

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*DOI: http://dx.doi.org/10.5772/intechopen.96549*

[11] Zimmet P, Alberti KG, Shaw J. Global and societal implications of the diabetes epidemic. Nature. 2001; 414:

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[14] Tuomilehto J, Lindstrom J, Eriksson JG, Valle TT, Hamallainen H, Ilanne-Pariikka, et al. The Finnish Diabetes Prevention Study Group: Prevention of type 2 diabetes mellitus by changes in lifestyle among subjects with impaired glucose tolerance. N. Engl. J Med 2001;

[15] The Prevention or Delay of Type 2

[16] Knowler WC, Barrett-Conner E, Fowler SE, Hammon RF, Lachin JM, Walker EA and Nathan DM. The Diabetes Prevention Program Research Group: Reduction in the incidence of

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Principles of Screening (draft). Geneva: World Health Organization, 2001

[17] World Health Organization.

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[2] Kasper DL, Fauci AS, Hauser SL, Lango DL, Jameson JL, Loscalzo J. Harrison's Principles of Internal Medicine. 17th ed. USA: The McGraw-Hill Companies, Inc; 2015. Chapter 338:

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[4] Diagnosis and Classification of Diabetes Mellitus. American Diabetes Association position statement. Diabetes

[5] Ramachandran A, Snehalatha C and Viswanathan V. Burden of type 2 diabetes and its complications-The Indian scenario. Current Science 83;

[6] IDF Diabetes Atlas. 88th Edition. Brussels: International Diabetes

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[10] Harris MI, Klein R, Wellborn TA, Kruiman MW. Onset of NIDDM occurs

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Federation. 2017

A1C values just above the upper limits of normal, depend upon post-prandial glycaemias (ie 2hPG), still requiring the OGTT to be correctly interpreted. Although in certain cases A1c gives equal or almost equal sensitivity and specificity to glucose measurement, it is not available in many part of the world and it is not well enough standardised for its use to be recommended at this time particularly in low- and mid-income countries.

Finally, although ADA recommended the use of A1c, by emphasizing the importance of IFG and IGT, which cannot be diagnosed without OGTT, to disclose high-risk subjects for diabetes, obviously shows that A1c would be of minute use without OGTT and that pre-diabetes includes different entities. OGTT is the only test that can efficiently disclose obscure diabetes when FPG<7.0 mmol/L and screen competently within the range of rather heterogeneous pre-diabetic values [120].

In conclusion an OGTT is undeniably the best test in investigation of dysglycaemia, either with the intention of testing for pre-diabetes, type 2 diabetes, or for gestational diabetes mellitus.
